1. Field of the Invention
The present invention relates to a heterodyne interferometer capable of measuring the velocity or vibration of an object under test accurately in a non-contact manner by utilizing the interference of light.
2. Description of the Prior Art
FIG. 7 shows the arrangement of a conventional heterodyne interferometer disclosed in publication "Optics and Laser Technology", Vol. 7, No. 1, pp. 11-16, by P. Buchhave, published in 1975 by IPC Science and Technology Press. In the figure, symbol 1 denotes a laser oscillator, 2 is a beam splitter, 3 is a frequency non-modulated beam, 4 is a frequency modulated beam, 5 is a mirror, 6 is a frequency modulator, 7a and 7b are converging lenses, 8 is a photodetector, 9 is an interference signal, and 10 is a frequency analyzer.
This heterodyne interferometer operates as follows. A laser beam produced by the laser oscillator 1 is split into two beams by means of the beam splitter 2. One beam as a frequency non-modulated beam 3 reaches the converging lens 7a without any rendition of frequency modulation, and another, beam as a frequency modulated beam 4 is reflected by the mirror 5 and is incident to the frequency modulator 6, where the beam is subjected to a small frequency modulation (with a frequency shift of f.sub.o) and the resulting beam reaches the converging lens 7a. Both beams 3 and 4 are focused by the converging lens 7a to a point P in a fluid including very small particles and moving at velocity V in the direction perpendicular to the optical axis (shown by the arrow in the figure). The beams 3 and 4 are reflected by particles and subjected to the Doppler shift depending on their frequencies.
In case the object under test is a transparent fluid, the beams 3 and 4 after the focal point P are collected by the converging lens 7b and are incident to the photodetector 8. The photodetector 8 integrates the amount of light received on its entire light-sensitive plane thereby to produce an interference signal 9 for the two beams 3 and 4. The interference signal 9 has a Doppler frequency f.sub.d expressed as follows. EQU f.sub.d =.vertline.F.sub.o -(2V/.lambda.) sin(.theta./2).vertline.(1)
where .lambda. is the wavelength of the laser beam emitted by the laser oscillator 1, and .theta. is the angle of intersection of the non-modulated beam 3 and modulated beam 4. Accordingly, the frequency analyzer 10 evaluates the velocity V of the object under test from the Doppler frequency f.sub.d of the interference signal 9 through the calculation of the above expression (1).
However, in the foregoing conventional heterodyne interferometer, the amount of light incident to the photodetector 8 varies due to the of the surface of particles included by the fluid at the moving focal point P of the laser beams, or in the multi-point measurement based on the scanning of the laser beams, and consequently the resulting interference signal fluctuates. In some cases, the level of the interference signal becomes too low to ensure the accuracy of measurement of the moving velocity of the object under test.